Add notes for an option

cherry-picked from aom/master:
2b407394

Change-Id: Ia7b3bfd68e2c31b21f49a429fecc4d0b701b045f

aomenc.c

@@ -376,8 +376,10 @@ static const arg_def_t qm_min = ARG_DEF(
 static const arg_def_t qm_max = ARG_DEF(
     NULL, "qm_max", 1, "Max quant matrix flatness (0..15), default is 16");
 #endif
-static const arg_def_t frame_parallel_decoding = ARG_DEF(
-    NULL, "frame-parallel", 1, "Enable frame parallel decodability features");
+static const arg_def_t frame_parallel_decoding =
+    ARG_DEF(NULL, "frame-parallel", 1,
+            "Enable frame parallel decodability features "
+            "(0: false (default), 1: true)");
 static const arg_def_t aq_mode = ARG_DEF(
     NULL, "aq-mode", 1,
     "Adaptive quantization mode (0: off (default), 1: variance 2: complexity, "